1. Field of the Invention
The present invention relates to a battery pack, battery powered device, and contactless charging method that magnetically couples a receiving coil in a battery pack placed on a charging pad with a transmitting coil in the charging pad to transmit power by magnetic induction and charge the battery housed in the battery pack.
2. Description of the Related Art
Portable battery powered devices such as mobile phones and portable audio devices that can be carried conveniently are most often powered by batteries that can be recharged. These types of battery powered devices house an individual battery or batteries, or a battery pack. A battery powered device is charged with the battery or batteries inside by physical connection to a battery charger. Meanwhile, a charging pad has been developed to charge the battery or batteries without connecting electrical contacts, but rather by transmitting power via magnetic induction from a transmitting coil housed in the charging pad to a receiving coil.
Refer to Japanese Laid-Open Patent Publication 2008-141940.
The charging pad 910 and battery powered device 920 cited in JP 2008-141940 and shown in FIG. 12 are provided with a transmitting coil 911 driven by an alternating current (AC) power source housed in the charging pad 910, and a receiving coil 921 in the battery pack 930 that magnetically couples with the transmitting coil 911. A rechargeable battery cell 931 in the battery pack 930 is charged by power induced in the receiving coil 921. The battery pack 930 contains charging circuitry that rectifies AC power induced in the receiving coil 921 and supplies it to the rechargeable battery cell 931 for charging. With this system, a battery pack 930 can be placed on the charging pad 910 and the rechargeable battery cell 931 can be charged in a wireless manner without connecting electrical contacts (contactless charging).
To determine charge conditions during contactless charging of the rechargeable battery cell housed in a battery powered device, techniques have been implemented such as monitoring rechargeable battery cell voltage and/or charging current. However, accurate determination of the remaining charge capacity of the rechargeable battery cell has been problematic. Further, from the perspective of the battery pack, it is not trivial to determine whether the battery pack is (the bare unit) alone or is connected with the battery powered device.
In a system that charges through a current detection resistor connected to measure remaining capacity, a scheme can be conceived to determine whether the battery pack is alone or connected with a battery powered device by whether or not current flow is detected in the current detection resistor. However, this type of current flow judgment is subject to the effects of offset voltage in the amplifier used to amplify current detection resistor voltage. In particular, when battery cell voltage is high, judgment becomes difficult because the charging current is low. In constant current-constant voltage charging systems as well, current flow judgment is difficult when current flow becomes low during constant voltage charging.
The present invention was developed with the object of resolving the prior art issues described above. Thus, it is a primary object of the present invention to provide a battery pack, battery powered device, and contactless charging method that can accurately determine during battery pack charging whether the battery pack is attached to a battery powered device or is the bare battery pack alone, and can accurately compute remaining battery capacity.